1. Field of the Invention
The present invention relates to colored, heat bleachable composition that changes its electromagnetic absorption characteristics upon application of heat. This colored, heat bleachable composition is useful, for example, as a filter layer or an antihalation layer in a photographic element, such as a photothermographic element. The composition is also useful for thermographic imaging.
2. State of the Art
A variety of photographic materials are known which can be processed by immersion in various processing solutions or baths. It is well known to provide different filter layers, including filter overcoat layers, filter interlayers and antihalation layers in such elements to provide improved photographic reproduction. The filtering capability or antihalation capability of these layers is generally removed during processing of these elements by one of the processing solutions or baths. This provides a processed element that is transparent to the desired region of the electromagnetic spectrum, typically the visible region.
Imaging elements are also known which can be processed, after imagewise exposure, simply by heating the element. These elements include known heat developable photographic elements, also known as photothermographic elements. It is desirable that heat developable elements, such as heat developable photographic films, have an antihalation layer or filter layer, especially to provide improved microimaging capability. In most cases, these filter layers or antihalation layers must be rendered substantially transparent upon heat processing in order to avoid use of processing baths or solutions.
The antihalation layer of a photographic element helps to prevent light that has passed through the radiation sensitive layer from reflecting back into the light sensitive layer. If this undesirable reflection is not prevented, the reflected light can reduce the sharpness of the resulting image. Antihalation layers, and filter layers, have been suggested for use with heat developable photographic elements.
Antihalation layers in heat developable materials are known which change from colored to colorless on exposure to heat or light. No separate activating component is used in such an antihalation layer. The dyes described provide antihalation protection; however, the dyes generally require higher temperatures than desired before they change from colored to colorless. Another antihalation layer is known in a heat developable photographic element that contains an acid component of a dye which is neutralized by a heat generated base. This is described, for instance, in U.S. Pat. No. 3,769,019 of Wiese et al. Decolorization of the described dye takes place by removal of an acid portion by heat. A problem with this antihalation material is that the decolorized form of the dye is not always as permanent as desired. The decolorized form changes back to a colored form which adversely affects the developed image. This can occur within an undesirably short period of time.
Photobleachable antihalation layers containing dyes that are photobleachable are also known. It is often disadvantageous, however, to have a photosensitive antihalation layer, as a post-process light bleaching step is needed, an additional processing operation.
Antihalation layers have also been used both for vesicular and diazo imaging films. These films, however, involve exposure of the antihalation layer to light to inactivate the material and do not use heat to bleach the antihalation layer.
Typically, layers that can be useful as antihalation layers, or filter layers, in a photothermographic element, can also be useful alone on a support to provide an element that is useful for thermographic imaging purposes. That is, the element can be imagewise heated to provide heat bleaching in the imagewise exposed areas. In this method of imaging any color change can be useful to form an image. For example, a substantially colorless dye precursor can be imagewise heated to form a colored image. A colored dye can also be imagewise thermally exposed to form a colorless material to provide an image. A variety of thermographic materials are known that can use this concept. Brief imagewise heating of the element causes migration of the reactants which results in decolorization in the imagewise heated area. Many thermographic elements are also useful for making reflex copies, writing with a heated stylus or imagewise exposure with a laser. Imagewise changing of a formazan dye from colored to colorless with a reducing agent is also known. This is described, for example, in Research Disclosure, October 1974, Item 12617, pages 12--30, published by Industrial Opportunities Ltd., Homewell, Havant, Hampshire, PO9 1EF, UK. There has been a continuing need, however, to provide new and improved combinations of materials for thermographic imaging with dyes. This need has been especially true for materials which are more permanently changed in color to provide more stable images.
A variety of hexaarylbiimidazole compounds are known in the imaging elements in which a colorless compound is photolytically changed to a colored compound in the imagewise exposed areas. Imaging materials which embody hexaarylbiimidazole compounds are known which involve what is described as a free radical imaging mechanism such as described in U.S. Pat. No. 3,390,994 of Cescon, issued July 2, 1968. Formation of a colorless layer is not described for a heat developable photographic element comprising a formazan dye with a hexaarylbiimidazole compound.
Each of the described elements having a heat bleachable layer, especially the photothermographic elements containing a heat bleachable filter or antihalation layer, have at least one of the disadvantages: (1) undesirably high temperatures are required to bleach the layer; (2) in many cases even when the dye is bleached it is not adequately stable and has a tendency to form undesired stain or discoloration on prolonged post process keeping; and (3) the choice of suitable dyes is undesirably limited for antihalation and filter layer purposes. A need has also existed for antihalation layers that provide a desired neutral color for use in photothermographic films.